The invention relates to a method of and an apparatus for identification of a monitoring signal at a base station in a radio telephone system, in which a number of monitoring signals having different frequencies are used for monitoring the quality of radio links between the base station and mobile stations.
In the Nordic Mobile Telephone (NMT) system, the quality of the radio link between a base station and a mobile telephone is monitored by means of special monitoring signals. The base station generates a monitoring signal having a desired monitoring signal frequency. The NMT system utilizes four monitoring signal frequencies at intervals of 30 Hz. The base station transmits a monitoring signal to a mobile telephone which returns the monitoring signal immediately to the base station, and the base station determines the quality of the used radio link, especially the signal-noise ratio, by means of the received monitoring signal.
The base station also measures the frequency of the received monitoring signal to ensure that measures the right mobile station. For identification purposes, it is sufficient to know whether the frequency is the right one or some of the other four possible frequencies. The base station has to identify the frequency of the monitoring signal it has received during one signalling frame, in practice, however, in about 100 ms.
In one prior art solution, the monitoring signal from the receiver of the base station is applied through bandpass and high-pass filters and control switches to a mixer which mixes the received monitoring signal with an injection signal generated by the base station and having a frequency which is at an interval of 60 Hz from the monitoring signal to be received. The output signal of the mixer is applied to a 60 Hz bandpass filter and further through rectifying means to a comparator circuit in which the obtained direct voltage is compared with a preset reference voltage. If the received monitoring signal is the right one, the direct voltage proportional to the signal exceeds the reference value and the state of the output of the comparator circuit is changed. If the received monitoring signal is not the right one, the rectified voltage does not exceed the value of the reference voltage, and the state of the output of the comparator circuit remains unchanged.
A drawback of this prior art method is the deterioration in the S/N ratio occurring in the mixing at low signal levels to be used when noise increases. The narrow 60 Hz bandpass filter accomplished with discrete components is sensitive to variation in component values and thus hampers the adjustment of the reference voltage level. In addition, this prior art arrangement is highly critical with respect to the field intensity, since the greater the amount of noise present in the received monitoring signal, the narrower the bandwidth (and the longer the rise time) required from the bandpass filter, keeping in mind, however, that the measuring time should not exceed 100 ms.